Handler

Remarks

A Handler can be easily used to execute code after a delayed amount of time. It is also useful for executing code repeatedly after a specified amount of time by calling the Handler.postDelayed() method again from within the Runnable's run() method.

HandlerThreads and communication between Threads

As Handlers are used to send Messages and Runnables to a Thread's message queue it's easy to implement event based communication between multiple Threads. Every Thread that has a Looper is able to receive and process messages. A HandlerThread is a Thread that implements such a Looper, for example the main Thread (UI Thread) implements the features of a HandlerThread.

Creating a Handler for the current Thread

Handler handler = new Handler();

Creating a Handler for the main Thread (UI Thread)

Handler handler = new Handler(Looper.getMainLooper());

Send a Runnable from another Thread to the main Thread

new Thread(new Runnable() {
public void run() {
// this is executed on another Thread
// create a Handler associated with the main Thread
Handler handler = new Handler(Looper.getMainLooper());
// post a Runnable to the main Thread
handler.post(new Runnable() {
public void run() {
// this is executed on the main Thread
}
});
}
}).start();

Creating a Handler for another HandlerThread and sending events to it

// create another Thread
HandlerThread otherThread = new HandlerThread("name");
// create a Handler associated with the other Thread
Handler handler = new Handler(otherThread.getLooper());
// post an event to the other Thread
handler.post(new Runnable() {
public void run() {
// this is executed on the other Thread
}
});

Stop handler from execution

To stop the Handler from execution remove the callback attached to it using the runnable running inside it:

Using a Handler to execute code after a delayed amount of time

Executing code after 1.5 seconds:

Handler handler = new Handler();
handler.postDelayed(new Runnable() {
@Override
public void run() {
//The code you want to run after the time is up
}
}, 1500); //the time you want to delay in milliseconds